1. Field of the Invention
The present invention relates to a position detecting device for detecting a positional relationship between a first component and a second component, comprising a marking arrangement which is provided on the first component and has an absolute track and an incremental track, the absolute track having a plurality of absolute markings from which a binary absolute code sequence can be read, and the incremental track having a plurality of incremental markings which are associated with the absolute markings and from which an alternating incremental sequence can be read, and comprising a sensor arrangement which is provided on the second component and is configured to scan the absolute markings and the incremental markings. The invention also relates to a method for producing a marking arrangement for a position detecting device and to a position detecting device having a marking arrangement produced by a method of this type.
2. Description of the Related Art
Position detecting devices and methods of the type mentioned above are used in the prior art, for example in machines and installations in which two components are arranged such that they can be moved or rotated relative to one another and information about the extent of the mutual movement or rotation of the two components is to be acquired. In this respect, a movement of the components can be a distance along a linear or curved displacement path or a rotational movement of the two components can be an angle of rotation or a curved length along a circular path. From the absolute markings, the sensor arrangement of known position detecting devices can read out information about the absolute extent of the movement or rotation of the two components (an absolute position) in any position of the marking arrangement, the accuracy of the absolute information being restricted to the distance between two adjacent absolute markings. To accurately detect the positional relationship, the sensor arrangement also scans the alternating incremental sequence of the incremental markings and establishes a relative position between two adjacent incremental markings. The exact positional relationship of the two components to one another can be determined from a combination of the relative position and the absolute position.
For known position detecting devices, marking arrangements and sensor arrangements based on the detection of magnetic fields are often used. In this respect, the absolute track and the incremental track can each be formed by a magnetised tape along the direction of movement of the two components, each tape comprising a plurality of magnetic poles which are arranged at equal distances along the running direction of the tape and at which the tape is magnetised in a vertical direction to the surface of the tape, the direction of the magnetisation encoding the binary value of the respective pole. A magnetic sensor arrangement, guided past the tape, of the other component can then read out the magnetisation of the individual poles and, for example, can recognise a detected north pole as signal “1” and a detected south pole as signal “0”.
FIG. 1 schematically shows a position detecting device of the prior art for detecting a positional relationship between a first component 110 and a second component 111, which components can be moved linearly relative to one another along a displacement path X. The position detecting device comprises a marking arrangement 112 mounted on the first component 110 and a sensor arrangement 113 which is attached to the second component 111 and reads out the marking arrangement 112. The marking arrangement 112 has an absolute track 114 with a plurality of absolute markings 116 which encodes a binary absolute code sequence B′={1, 1, 0, 0, 1, 0, 1, 1, 1, 1, 0, 1, 0, 0, 0, 0}. Also belonging to the marking arrangement 112 is an incremental track 118 with a plurality of incremental markings 120, the incremental track 118 encoding an alternating incremental sequence I′={1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0}. The absolute track 114 and the incremental track 118 run parallel to one another and the respective successive markings 116, 120 thereof are at the same intervals to one another in each case.
The sensor unit 113 comprises an absolute sensor array 122 with four absolute sensors 124 which can simultaneously read out a sequence of four successive absolute markings 114. Also provided in the sensor unit 113 is an incremental sensor 126 which detects the incremental marking 120 and provides a signal corresponding to a position between two adjacent incremental markings 120.
The absolute code sequence B′ of the absolute track 114 is encoded on the basis of a De Bruijn sequence of order 4.
The term “De Bruijn sequence of order N” is understood as meaning a cyclic sequence of characters of a predetermined set of characters A in which each subsequence of length N, which can be obtained from the set of characters A, is contained precisely once. For the purpose of this definition, subsequences are read with the proviso that the last character of the De Bruijn sequence is again followed by the first character of the De Bruijn sequence. Accordingly, the cyclical characteristic of the De Bruijn sequence is expressed in the fact that the sequence can be divided up anywhere into two subsequences, and after interchanging the two subsequences (adding the first subsequence at the end of the second subsequence), a De Bruijn sequence of order N is again achieved according to the above definition. In other words, the De Bruijn sequence can be shifted cyclically to the right or to the left and, during each shift by one place, a character arranged at one end of the De Bruijn sequence is removed and is added to the opposite end of the De Bruijn sequence (cyclic shift by one place). During each cyclic shift of the De Bruijn sequence, a De Bruijn sequence of the same order N is again produced.
For the present use of marking arrangements with a binary absolute code sequence, De Bruijn sequences with a binary set of characters are of interest in which either the value “0” or the value “1” is associated with each place of the De Bruijn sequence, i.e. with each marking.
Binary De Bruijn sequences of order N have a number of places of 2N so that the length L of an absolute track formed thus is given byL=p·2N,where p represents the pole pitch, i.e. the distance along the absolute track between two adjacent absolute markings. Thus, with a sensor unit for detecting N successive absolute markings, it is possible to scan an absolute track, the absolute markings of which encode a De Bruijn sequence of order N, in which case the movement, which can then be maximally detected by the position detecting device, between the two components is given by the length L, to be calculated as above, of the absolute track.
In order to increase the maximum distance, which can be detected by a known position detecting device, of the relative movement between the two components, it might be possible to increase the order N of the De Bruijn sequence, stored in the absolute track, to increase the length L of the absolute track in accordance with the above-mentioned equation. However, this would increase the length of the sensor arrangement in the direction of the displacement path, since, according to the conventional detection principle, the sensor arrangement has to be configured for scanning N successive absolute markings. Alternatively, it might be possible to increase the distance between successive markings (pole pitch p). In this case as well, at least the installation space of the sensor arrangement would have to be increased accordingly to be able to read out N successive absolute markings, and on the other hand, with a solution of this type, the accuracy of detection of the relative position on the basis of the incremental markings would be diminished.
It is therefore the object of the present invention to provide a position detecting device for detecting a positional relationship between two components as well as a method for producing a marking arrangement for a position detecting device, using which device and method it is possible to detect a positional relationship between two components, arranged movably relative to one another, over a relatively long total distance, without having to accept significant disadvantages in respect of the cost and installation space of the position detecting device or in respect of the accuracy of the detection.